# Why removing one slab in Michelson-Morley experiment causes an elliptical fringe pattern?

I performed Morley experiment using He-Ne laser. Two glass slabs were there and circular fringes were formed but when I removed one slab elliptical fringes were there (and less intense fringes). Why?
In general is there any benefit of using two slabs?

• The general answer to question like this concerning interferometers is "follow the optical path down various branches; you want to make those paths as similar as possible". – dmckee --- ex-moderator kitten Oct 27 '13 at 16:32

Beam #1 bounces off the front of the beam splitter, goes over and off the front of a mirror, comes back and goes through the beam splitter at a $45^{\circ}$ angle. (One pass through the glass of the beam splitter)
Beam #2 goes through the beam splitter, over to the other mirror, comes back and goes through the beam splitter from behind at a $45^{\circ}$ angle, bounces off the back of the half-silvered surface, and goes back through the beam splitter again at a $45^{\circ}$ angle. (Three passes through the beam splitter glass)
Adding a clear piece of the beam splitter glass at a $45^{\circ}$ angle to the first path makes the two beams optically equivalent.